XLPE stands for Cross linked Polyethylene.  It is a long chain polymer whose chains are cross-linked.  XLPE is a widely used material for use in cable insulation.  XLPE can be used from LV cables to cables with voltages upto 275 kV. 

The power factor of XLPE cable is very close to one.  XLPE cables have smaller charging currents and lesser dielectric loss.

XLPE cables do not require the protective metallic sheath.  They are lighter as compared to other cables and are easier to lay.

Copper used in bus bars is sometimes plated with Tin.  This is because copper oxidises in the presence of air.  The oxides of copper fall off the surface, this exposes more copper and further oxidation takes place.

These oxides increase the contact resistances in electrical contacts causing localized heating.  This heating causes further oxidation.  Thus, the copper is steadily eroded.

Tin plating is used protect the copper from corrosion.   Tin does not react easily with either air or water.  Tin is a soft metal which can be easily polished to give a shiny finish.   Tin Plated contacts are also used in the contacts of  isolators and switches.

Flexible bus bars are used in Low voltage installations.  The are used in place of cables, particularly where parallel runs of cables need to be drawn to supply equipment in a particular location. 

Flexible bus bars are made of flexible strips of electrolytic copper which is usually tin-plated.  These strips are enclosed in a PVC insulation.  These bus bars can slide over each other which makes them flexible.  These bus bars can be bent to a considerable extent.  The elimination of parallel runs of cables reduces costs. 

Flexible bus bars have better heat dissipation than conventional rigid bus bars.  Hence, they can carry higher currents for the same cross section. 

Since these bus bars are insulated, the clearance from the ground can be lesser as compared to normal bus bars. 

Oil is an important component of most transformers.  Oil serves to dissipate the heat generated in the core.  It also serves to provide dielectric isolation between the windings and the transformer body.  Transformer oils need to be stable at high temperatures with excellent dielectric properties.  

Traditionally,  mineral oils have been used in transformers. The downside of mineral oil is that they are easily combustible causing transformer fires. 

Polychlorinated Biphenyls, a synthetic compound was used as a replacement for mineral oil as it is not inflammable.  However, it is highly toxic and carcinogenic, not biodegradable and can cause damage to the environment if leaked.   The use of PCBs is now banned in many countries. 

Research has led to the development of many types of transformer oil which are fire-resistant and non-polluting.  Some of these oils are based on esters which are naturally available in vegetable oil or on synthetic esters.  These oils though expensive are environment-friendly, fire resistant and have the require dielectric properties.  Their high cost is offset by the elimination of the need for building expensive vaults for the transformer to contain any fire.